It is often desirable to inject a treatment fluid into a crude oil and/or natural gas production well. The treatment fluid may contain a corrosion or hydrate inhibitor, a viscosity reducing agent, a chemical agent to prevent deposition of sulphur, asphaltenes or waxes, or a foam generating agent which inhibits accumulation of condensed water and/or condensates downhole in the well.
Many oil and/or natural gas production wells are equipped with a Surface Controlled Subsurface Safety Valve (known as a SCSSV) which may be suspended at about 100 m depth in the production tubing. A SCSSV automatically closes if the hydraulic pressure in the SCSSV hydraulic control conduit falls away, for example as a result of a power supply interruption at the oil and/or gas production facilities at the earth surface, impact on the wellhead from outside or an emergency shut-down action.
U.S. Pat. No. 4,042,033 suggests a method to inject a chemical fluid into the production tubing above a SCSSV via the hydraulic control conduit of the SCSSV, which is equipped with a chemical injection opening just above the SCSSV. The injected chemical is used to provide hydraulic pressure to keep the SCSSV open, and is bled off into the production tubing above the SCSSV via a one-way check valve that is mounted in the upper part of SCSSV assembly.
A problem associated with the known SCSSV assembly is that it does not allow to inject chemicals into sections of the well below the SCSSV. The SCSSV assembly blocks the possibility of suspending a chemical injection conduit from the wellhead into the production tubing below the SCSSV, as the chemical injection conduit would form an obstruction to closure of the SCSSV in an emergency situation.